Mozilla is planning to retire Firefox 3.6 from support, but won't put the 18-month-old browser out to pasture until August at the earliest.

Mozilla is planning to retire Firefox 3.6 from support, but won't put the 18-month-old browser out to pasture until August at the earliest.

The retirement of Firefox 3.6 would follow that of Firefox 4 and Firefox 3.5 earlier this year.

Firefox 3.5, which Mozilla launched in mid-2009, received its last security update in April. Since then, users have been encouraged to upgrade to either Firefox 3.6 or 4. Going forward, the estimated 12 million users of Firefox 3.5 will see their browser automatically updated to Firefox 3.6 , a move the company discussed last month.

That move has been criticised by some enterprise IT managers . They've argued that the quick retirement combined with Mozilla's rapid-release scheme -- which delivers a new edition every six weeks -- puts them in an impossible position: By the time they test one version, another will already be out.

Mozilla has decided to issue at least one more security update for that edition, which shipped in January 2010. The update, to be pegged 3.6.19, will be released alongside Firefox 6 when the latter ships on Aug. 16.

The company has not disclosed other parts of its plan to declare Firefox 3.6 at "end of life," or EOL. Although Mozilla held a meeting Tuesday in its Mountain View, Calif. office to discuss Firefox 3.6's retirement, unlike its usual practice the company did not publish notes from that meeting.

The demise of Firefox 3.6 from support would put users who have stuck with the older version in a bind.

IBM, for example, selected Firefox 3.6 last summer as its default browser for employees.

John Walicki, the manager of workplace and mobility in the office of IBM's CIO, has complained about the retirement of Firefox 4, saying that it would force him to make a tough decision.

"I'm now in the terrible position of choosing to deploy a Firefox 4 release with potentially unpatched vulnerabilities, reset the test cycle for thousands of internal apps to validate Firefox 5 or stay on a patched Firefox 3.6.x," Walicki said in a comment appended to a blog post .

When Mozilla retires Firefox 3.6, that last option won't be available to Walicki and IBM.

Last week, Ari Bixhorn, director of IE, published an open letter to Walicki urging him to consider switching from Firefox to IE8 or IE9. "I think I speak for everyone on the IE team when I say we'd like the opportunity to win back your business," said Bixhorn.

The retirement of Firefox 3.5 will affect only a small fraction of Mozilla's active users. According to Web metrics company Net Applications, Firefox 3.5 accounted for just 6% of all copies of Mozilla's browser used in May.

It's unclear how many users would be impacted in mid-August by the retirement of Firefox 3.6: Although that edition accounted for 42% of all copies last month, the browser has been on a steady decline as people have upgraded to Firefox 4. If the trend of the last three months continues, Firefox 3.6 would near extinction by the end of August.

Firefox 3.6.18 can still be downloaded manually from Mozilla's website.